The use of indicators to measure the volume of liquid in tanks is well known. Such indicators are typically of the invasive type, in that they are constructed to measure the volume or level of liquid in a tank through the use of one or more members that resides within the tank itself. Such invasive-type of liquid measuring device can include one or more members that is also disposed outside of the tank. Invasive measuring devices are adequate for use in applications where the tank is either not covered, or is covered but not pressurized.
Certain types of liquid handling containers are, however, pressurized for the purpose of performing a particular function. An example of such liquid handling container can be provided in the form of a hydraulic surge suppressor that comprises a pressurized diaphragm therein for the purpose of accommodating a pressure surge or hydraulic transient in the fluid line that it is attached to. In such an example application it is intended that the gas within the diaphragm be maintained under substantial pressure. For this reason, it is desired that there be no intrusive means for measuring the gas-to-liquid ratio, hence no perforations or holes in the walls of the tank, except for the inlet and outlet ports. This eliminates such obvious expedients as a sight glass, for example.
In U.S. Pat. No. 3,312,107, a method is taught for determining the amount of liquid in a tank by providing a swept frequency signal to excite an electromagnetic field within a metallic storage tank containing the liquid to be measured. The tank behaves as a resonant transmission type of cavity. When the sweep frequency signal equals one of the resonant frequencies of the tank, the signal is transmitted through the tank where it is detected at the output. Implementation of this measuring technique involves inserting a small coupling loop through the wall of the tank for exciting an electromagnetic field within the tank. A detector, such as a crystal, is coupled to the tank by means of another small coupling loop. The use of this device and method for measuring, of course, involves penetrating the wall of the tank at two points other than the inlet and outlet ports, which presents unwanted sealing problems.
In addition to the above requirements, for some locations it is desirable that the measuring instrumentation be sufficiently portable that it can be moved around to measure the liquid contents of a number of tanks. Alternatively, all or part of the instrumentation may be at least semi-permanently attached to the tank.
It is, therefore, desired that a device be constructed that is capable of providing an accurate measurement of liquid level within a liquid container in a non-invasive manner. It is desired that such device be capable of providing such volume measurement in volume containers that may or may not comprise a pressurized diaphragm or other type of pressurized member therein. It is further desired that such liquid volume device be somewhat portable so that it can be used to measure the liquid volume in more than one liquid container.